Sealing components and gasket components used in electrical appliances or the like are essentially required to be excellent in terms of barrier properties, sealing properties etc., to satisfy a desired hardness, to have high heat resistance for withstanding heat generated during the flow of electric current, and to be usable even in low-temperature environments.
For example, fuel cells are efficient and clean power generation systems, in which electricity is directly produced by the reverse reaction of electrolysis of water, i.e., by allowing hydrogen and oxygen to chemically react with each other, and the system attracts attention as novel energy systems for automobiles and household appliances. Cell sealing members for fuel cells or the like have required a low-cost material excellent in terms of heat resistance, acid resistance, gas permeation resistance, and high-speed moldability. Under present circumstances, a fluororubber is used in terms of heat resistance and acid resistance, a butyl rubber is used in terms of gas permeation resistance, and a silicone rubber is used in terms of heat resistance and moldability. However, when the high-speed moldability is required, normal millable silicone rubbers are insufficient to meet the requirement. In such a case, for example, a liquid silicone rubber is used and liquid injection molding (LIM) is applied. Silicone rubber is excellent in terms of heat resistance and high-speed moldability, but is insufficient in terms of acid resistance and gas permeation resistance.
Furthermore, there has been studied for increasing the power generation reaction temperature of fuel cells in order to further improve power generation performance. Accordingly, sealing members (gasket members) also require higher high-temperature durability.
Meanwhile, with the reduction in the size and increase in performance of electronic devices, reductions in the size and thickness of components constituting such products have been desired. However, the reduction in the size of the components degrades assembly workability in manufacturing. Therefore, integration and combination of various components have been desired.
For example, a gasket for a hard disk drive, which is an electronic storage device, is often used in a form that a simple rubber or a urethane foam sheet and a metal cover, such as a stainless steel cover or an aluminum cover, are bonded with an adhesive to be integrated. However, not only the above integration process but also reductions in the weight and thickness of the metal cover are implemented, and therefore the gasket with a high hardness (reaction force) causes a problem of deformation of the cover.
Under these circumstances, a styrene thermoplastic elastomer has been proposed as a gasket material (Patent Document 1). The document describes that such a styrene thermoplastic elastomer has a low hardness and does not need a vulcanization process differently from rubber materials, and thus the production process can be simplified and the elastomer can be recycled.
However, the elastomer is often applied to hard disk drives involving heat generated due to an increase in the performance (high rotational speed) or automobiles, and tends to be used under high-temperature environments, for example, at 80° C. or higher. Such environments caused a problem of permanent set at high temperatures, which is one of mechanical properties of the styrene thermoplastic elastomer. This showed a limitation in terms of the performance of the elastomer. Consequently, it is desirable to realize a gasket member which has further improved high-temperature durability, low hardness, and low compression set ratio, and which exhibits sealing properties with a low reaction force even under high compression.
Electric wire connectors are used for connecting and branching electric wires, and comprises a pair of male and female resin frames that can be connected to each other through one-touch operation, an electric wire, and a sealing member. The sealing member is mainly used as a dust seal between the electric wire and the resin frame. For the electric wire connector sealing member used in this type of connector, sealing properties and insertion properties for thin electric wires are required, and heretofore, low-hardness and oil-bleeding type silicone rubber and nitrile rubber have been used. However, these rubbers mainly contain silicone oil as a plasticizer, and thus the plasticizer adheres to electrical contact points during the use, thereby causing troubles in the flow of electric current due to insulation at the electrical contact points.
Under these circumstances, Patent Document 2 proposes a rubber composition which is excellent in terms of high-speed moldability and excellent in heat resistance, acid resistance, and gas permeation resistance, and which is suitably used in a sealing member for a fuel cell, a gasket member for an electronic device such as a hard disk drive, a sealing member for an electric wire connector, and the like. This rubber composition is a liquid rubber composition that can be processed by LIM molding, and can provide moldings having a low hardness. Accordingly, the rubber composition is suitable for a fuel cell sealing member, a gasket member for hard disk drives, and the like, for which sealing properties with a low reaction force under high compression are also required. However, a further improvement in high-temperature durability has been desired for moldings such as gaskets and sealing components, and improvements not only in the compression set properties at high temperatures but also in unsusceptibility to compression cracking under high temperatures and high strain have been desired. However, such a rubber composition is inferior to silicone rubbers and the like in terms of elastic recovery ratio in low-temperature environments, and therefore, a further improvement has been desired in the mechanical properties in low-temperature environments.
A sealing member for stationary fuel cells for household use has required a compression set ratio of 80% or less after standing at 90° C. for 40,000 hours. However, the sealing member has a problem that the compression set ratio significantly deteriorates from 5,000 hours to 10,000 hours. Accordingly, a rubber composition having a further improved long-term compression set ratio at high temperatures has been strongly desired.    Patent Document 1: Japanese Patent No. 2961068    Patent Document 2: International Publication WO03/057777